The present invention relates to a device for feeding a gas, particularly air, into a liquid contained in an open receptacle and also relates to method of agitating the liquid by such a device.
The injection of a gas into a liquid present in an open container such as for example a galvanizing or electroplating bath, is known since long time. Even if this liquid agitating method cannot be employed in all kinds of galvanizing baths, in principle it has proved to be very advantageous. Due to the injection of air an intensive agitation of the bath results.
It is conventional to introduce air in the bath in such a manner that a perforated pipe is arranged below and parallel to a rod on which articles to be electroplated are suspended. The perforations in the air injection pipe are provided along its longitudinal axis in one or more rows. The air injection pipe is connected to an air supply conduit for feeding in air at higher pressure than the ambient air pressure. The pressure difference must be at least such as to overcome the pressure of the liquid column around the air injection pipe.
Air exiting from the air injection pipe ascends in the liquid and causes the same to circulate. Since, as mentioned before, the air injection pipe in the galvanizing bath is arranged below the supporting rod for the articles, a relative movement between the bath and the articles is generated. This relative movement has the advantageous affect that fresh bath solution or electrolyte is continuously supplied to the upper surface of the treated articles.
If the bath is not agitated, the electrolyte exchange occurs very slowly. For this reason, during the electroplating process the composition of the electrolyte in the region adjoining the upper surface of the treated articles is subject to change. For example, in the boundary region there is less amount of metal ions inasmuch they are continuously deposited on the electroplated articles. This phenomenon is called the "impoverishment" of the electrolyte in this region. As a sequence the galvanizing current by means of which the metallic coating is deposited must be kept low in order to obtain the desired quality of the deposited coating. Since the applicable current density is low, it takes a relatively long time until a coating with a desired thickness is obtained. However, in order to reduce the number of galvanizing cells needed for the electroplating of a given upper surface of the treated articles, it is desirable to achieve the lowest electroplating time as possible. The agitation of the galvanizing bath by means of injected air is a measure for achieving this objective.
In conventional embodiments of the air injecting devices, it has been found to be difficult to maintain a uniform distribution of air exiting from the air injection pipe.
The uniformity of distribution of the exiting air must fulfill high requirements inasmuch as the uniformity of the deposited metal layer depends directly thereon. Especially in the case of electronic components such as for example printed circuit boards, very narrow tolerances as regards the thickness of the deposited layers are permissible.
There are different reasons for which a uniform distribution of the exiting air in the prior art devices, is rendered difficult. It has been necessary to maintain the air injection pipe, namely the row of the air outlet bores thereof, in an absolutely horizontal position. Furthermore, the pipe must have been stiff in order to avoid its sagging when the device is not in operation becasue when the pipe is bent, the liquid which enters the pipe is nonuniformly expelled when the device is put into operation. On the other hand, if the pipe is filled with air, the resulting uplift may even bend the pipe upwards.
To achieve a substantially uniform distribution of the air discharge over the entire length of the pipe it is conventional to provide the outlet bores at the lower side of the pipe. In this manner, when the operation is started volume of the pipe is filled with air before the latter starts uniformly exiting through the bores. This desired affect, however, can be achieved only then when the cross section of the pipe, the diameters of the bores and the distribution and number are selected in an optimal harmony. Since there is also necessary to meet other requirements for example the bores must not be made too small because resulting air bubbles would too small and would stick to the upper surface of the treated articles, it is difficult to find a solution in conventional devices of this kind which would satisfactorily meet all these requirements.